FIG. 1 illustrates a optical system having a conventional optical pickup which is used in a compact disk player. Specifically, the pickup comprises a printed circuit board (PCB) 1, a submount 2, an edge-emitting laser diode 3, a photodetector 4, and a photo diode 8. The submount 2 is mounted on the PCB 1, and the laser diode 3 is fixedly disposed on the side of the submount 2 and used as a light source for the optical pickup. The photo diode 8 is disposed on the PCB 1 next to the submount 2, intersects a line which is collinear with the optical axis of the edge-emitting laser diode 3, and senses the amount of light output from the diode 3. The photodetector 4 is also installed on the PCB 1 and detects light which has been emitted from the diode 3 and reflected from an optical disk 7.
The optical pickup further comprises a hologram device 5 and an objective lens 6. Specifically, the hologram device 5 and the objective lens intersect the optical axis of the diode 3 and are located between the diode and the disk 7.
The operation of the conventional optical pickup will be described below. First, the laser diode 3 emits a laser beam which has multiform polarization directions, and such beam impinges upon the hologram device 5. The device 5 converts the laser beam into a parallel light beam and outputs such beam to the objective lens 6. Then, the lens 6 focuses the light beam upon a particular track of the disk 7.
The focused beam reflects off the track of the optical disk 7, and the polarization of the light beam varies according to information recorded in the track. Subsequently, the reflected beam passes through the objective lens 6, is converted into a parallel beam, and is output to the hologram device 5. Then, the device diffracts the reflected beam and outputs the diffracted beam to the photodetector 4. Consequently, the photodetector 4 determines the type of information recorded on the track of the disk 7 based on the shape and/or intensity of the diffracted beam. Furthermore, the photodetector 4 evaluates the beam to determine if the beam is properly positioned on the track of the disk and if the beam is properly focused on the disk 7.
The conventional optical pickup has several disadvantages. For example, since the edge-emitting laser diode 3 is fixed to the submount 2, aligning the optical axis of the diode 3 with the hologram device 5 and aligning the photodetector 4 with the diffracted beam is difficult. As a result, a substantial amount of time is need to assemble the optical pickup.
Furthermore, the edge-emitting laser diode 3 emits a laser beam which has an oval cross section and a large astigmatism. Therefore, correctly detecting tracking errors via the photodetector 4 is difficult. Moreover, the intensity of the light reaching the surface of the disk significantly decreases as the laser beam passes through the optical system.